mvme5500: data transfer rate to and from vme space

Till Straumann strauman at
Wed Oct 24 07:04:42 UTC 2012

On 10/24/2012 01:41 AM, rwas wrote:
> Till Straumann wrote:
>>   Are you really using VME BLTs
> The answer is "no". Somehow I had it in my head that I was.
> I appologize.
>>   (or only repeated single-address
>>   cycles, e.g., the CPU doing a loop - this can be really slow,
> Yes, this is what I am doing.
>>   especially if the universe is configured so that the bus has to be
>>   re-arbitrated for for each cycle)?
> Looking at BR3, clearly it is. I was under the impression once a master
> took the bus, it kept it until someone else asked for it. Clearly wrong.
That is something you can set in the universe; RTM. Boils down
to a classical throughput vs. response-time tradeoff. (If you hold
the bus longer then throughput gets better but it takes longer
for somone else to get hold of the bus).
> It is clearly doing an address cycle every word.
> I put my scope on the backplane and  I'm getting an AS assertion with
> every DS.
Yeah. When you use the CPU then you can't do a block transfer
> All this with a BR3 for each cycle.
> Again I apologize, this is a pretty newb kind of error.
> Do you have any suggestions? Seems like there is no direct RTEMS way to
> make use of dma.
There is: <bsp/VMEDMA.h> You can specify the address mode
the DMA should use and there you can use a BLT mode.

- Till
> Thanks.
> Robert W.

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